Conventional magnetic disk drive designs comprise a commonly denominated Contact Start-Stop (CSS) system commencing when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions allowing data to be recorded on and retrieved from the surface of the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the stop and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk to be positioned in close proximity with an attendant increase in predictability and consistent behavior of the air bearing supporting the head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by techniques generally referred to as "texturing." Conventional texturing techniques involve polishing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers. The texturing treatment can be applied to the underlayer rather than the substrate.
A typical longitudinal recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-alloy, such as an aluminum-magnesium (Al--Mg)-alloy, plated with a layer of amorphous nickel-phosphorous (NiP). Alternative substrates include glass, ceramic and glass-ceramic materials and graphite. Substrate 10 typically contains sequentially deposited on each side thereof an underlayer, such as chromium (Cr) or a Cr-alloy 11, 11', a magnetic layer, such as a cobalt (Co)-base alloy 12, 12', a protective overcoat 13, 13', typically containing carbon, and a lubricant topcoat 14, 14'. Cr underlayer 11, 11' can be applied as a composite comprising a plurality of sub-underlayers 11A, 11A'. Cr underlayer 11, 11', Co-base alloy magnetic layer 12, 12' and protective carbon overcoat 13, 13' are usually deposited by sputtering techniques performed in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface for polishing to provide a texture, which is substantially reproduced on the disk surface.
The escalating requirements for high areal recording density impose increasingly greater requirements on thin film magnetic media in terms of coercivity, stiction, squareness, low medium noise and narrow track recording performance. In addition, increasingly high density and large-capacity magnetic disks require increasingly smaller flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive. The requirement to further reduce the flying height of the head renders it particularly difficult to satisfy the requirements for controlled texturing to avoid head crash.
Conventional techniques for providing a disk substrate with a textured surface comprise a mechanical operation, such as polishing. See, for example, Nakamura et al., U.S. Pat. No. 5,202,810. Conventional mechanical texturing techniques are attendant with numerous disadvantages. For example, it is extremely difficult to provide a clean textured surface due to debris formed by mechanical abrasions. Moreover, the surface inevitably becomes scratched during mechanical operations, which contributes to poor glide characteristics and higher defects. In addition, various desirable substrates are difficult to process by mechanical texturing. This undesirably limiting facet of mechanical texturing, virtually excludes the use of many materials for use as substrates.
An alternative texturing technique to mechanical texturing comprises the use of a laser light beam focused on an upper surface of a non-magnetic substrate. See, for example, Ranjan et al., U.S. Pat. No. 5,062,021, wherein the disclosed method comprises polishing an NiP plated Al substrate to a specular finish, and then rotating the disk while directing pulsed laser energy over a limited portion of the radius, to provide a textured landing zone leaving the data zone specular. The landing zone comprises a plurality of individual laser spots characterized by a central depression surrounded by a substantially circular raised rim.
Another laser texturing technique is reported by Baumgart et al. "A New Laser Texturing Technique for High Performance Magnetic Disk Drives," IEEE Transactions on Magnetics, Vol. 31, No. 6, pp. 2946-2951, November 1995.
In copending application Ser. No. 08/666,374 filed on Jun. 27, 1996 a laser texturing technique is disclosed employing a multiple lens focusing system for improved control of the resulting topographical texture. In copending application Ser. No. 08/647,407 filed on May 9, 1996, a laser texturing technique is disclosed wherein a pulsed, focused laser light beam is passed through a crystal material to control the spacing between resulting protrusions.
In copending PCT application Ser. No. PCT/US96/06830, a method is disclosed for laser texturing a glass or glass-ceramic substrate employing a laser light beam derived from a CO.sub.2 laser source. The textured glass or glass-ceramic substrate surface comprises a plurality of protrusions which extend above the substrate surface, without surrounding valleys extending substantially into the substrate as is characteristic of a laser textured metallic substrate. The effect of laser parameters, such as pulse width, spot size and pulse energy, and substrate composition on the protrusion or bump height of a laser textured glass or glass-ceramic substrate is reported by Kuo et al., in an article entitle "Laser Zone Texturing on Glass and Glass-Ceramic Substrates," presented at The Magnetic Recording Conference (TMRC), Santa Clara, Calif., Aug. 19-21, 1996.
It is recognized that laser texturing of alternate substrates such as glass, ceramic and glass-ceramic materials, is attendant upon several problems, notably microcracking. EPA 0652554 A1 addresses such a microcracking problem by controlling the radiant energy fluence during laser texturing so that it is less than the thermal shock threshold for the particular material undergoing laser texturing.
In copending application Ser. No. 08/796,830 filed on Feb. 7, 1997, a method is disclosed for laser texturing a glass or glass-ceramic substrate, wherein the height of the protrusions is controlled by controlling the quench rate during resolidification of the laser formed protrusions. One of the disclosed techniques for controlling the quench rate comprises preheating a substrate, as by exposure to a first laser light beam, and then exposing the heated substrate to a focused laser light beam. In copending application Ser. No. 08/919,100 filed Aug. 28,1997 (Our Docket No. 2674-054), a laser texturing technique is disclosed employed a dual fiber delivery system to separately control nucleation and crystallization growth rate, thereby avoiding thermal shock to an alternate substrate comprising a glass, ceramic or glass-ceramic substrate.
It is inevitable that the surface of a magnetic recording medium exhibits unevenness or waviness. In the magnetic recording industry, such lack of topographical uniformity is called "runout." The acronym "RVA" is employed in the magnetic recording industry to denote "Runout Velocity Acceleration," describing the motion of a rotating body as it deviates from an ideal plane of reference. In rigid disk and drive manufacturing, the two important reference directions are the axial direction which is parallel to the long axis of the spindle motor and perpendicular to the plane of rotation of the disk, and the radial direction which is perpendicular to the long axis of the motor and in the plane of rotation of the disk.
As flying heights are reduced lower and lower, the dynamics and limitations of the disk surface mechanical parameters become more and more critical. This problem becomes particularly acute in laser texturing, wherein topographical variations due to surface runout, particularly due to clamping of the periphery of the disk substrate, exceeds the depth of focus capabilities of conventional lens focusing systems employed in laser texturing.
There is a continuing need in the magnetic recording media industry for an efficient method and apparatus for uniformly texturing a magnetic recording medium to obtain an accurate controllable pattern of protrusions enabling lower flying heights. There exists a particular need for an efficient method and apparatus for laser texturing a magnetic recording medium, wherein the depth of focus of the lens focusing system is greater than the topographical variations due to surface runout.